The history of the universe — from the Big Bang to the end of the year — day by day

When Life Almost Died

The most famous mass extinction is the one that killed the dinosaurs. (Check it out on Logarithmic History, April 6). But the biggest mass extinction is the one at the end of the Permian, marking the transition from Paleozoic to Mesozoic Eras.

We’ve known for a generation now why the dinosaurs went extinct, but the cause of the Permian extinction is still disputed. Very likely changes in the atmosphere were central. Siberia saw volcanic eruptions putting massive amounts of CO2 into the air. This meant that oceans grew more acid, as carbon dioxide dissolved in the water. It may also have caused a change in ocean circulation. Nowadays, cold oxygen-rich water near the poles sinks to the depth and is carried by deep currents to the tropics. But in the end-Permian times, this conveyor belt may have stopped working, and stagnant waters may have grown anoxic—lacking oxygen. Today the Black Sea is like this: no oxygen and only microbial life below about 600 feet. (Check out the flooding of the Black Sea 7800 years ago on Logarithmic History, September 14.)

Something even more alarming may have happened at the end of the Permian. As anoxic conditions reached closer to the surface, life in the seas may have been taken over by sulfur-reducing purple bacteria producing hydrogen sulfide as waste. Hydrogen sulfide could have poisoned large areas of ocean, and bubbled into the atmosphere to poison land life as well. And it would have broken down the ozone layer to expose the Earth to dangerous levels of ultraviolet light. (This is called the Kump hypothesis, after Lee R. Kump)

The much shorter time scales of human history have also seen their share of catastrophes and civilizational collapses. We’ll have the chance, as Logarithmic History moves into Fall, to consider how much these had to do with environmental disasters.